As FBI officials worked to identify a suspect in the Boston Marathon bombing, parallel investigations were being conducted on the Internet, where ­amateur sleuths have been identifying their own cast of potential culprits.

There’s the guy with the dark backpack. The man with mangled pants running from the blast. The shadowy figure on the roof of a Back Bay building, moments after the explosion.

On Internet message boards and ­viral image websites, do-it-yourself detec­tives have taken up the mantle of investigating, uploading photos from the moments before and after the explo­sions and analyzing them for clues on who might be responsible.

“There has to be hundreds, if not thousands, of photographs, videos, and other observations that were made down at that finish line yesterday,” ­Timothy Alben, superintendent of the Massachusetts State Police, said at a press conference Tuesday, in asking for the public’s images. “You might not think it’s significant, but it might have some value to this investigation.”

Because the explosions occurred in the most-photographed stretch of the race, some civilians hope that the key to solving the investigation may come from one of those iPhone snapshots or stills from amateur video, and they ­believe their guesswork may unearth that critical clue.

Reddit’s “Find Boston Bombers” ­forum already has more than 1,700 users, who have highlighted suspicious persons and objects through collected photographs and videos. On ­Imgur.com, dozens have uploaded snapshots of the crowd close to the 26-mile marker.

In bold red circles, Internet commenters identified ominous black backpacks and drew yellow arrows to highlight individuals in the crowd perceived as shifty. With annotations added, they pointed out bystanders who appeared to be alone, using a cellphone, appearing to pay no attention to the finish line, or carrying a bag that could be big enough to hold a bomb.

“This guy is bent over looking through a bag near the area where the bomb detonated,” wrote one.

Hunting for clues in crowd photos in the wake of a national disaster is not a new phenomenon, said Mark Fenster, professor of law at the University of Florida, and author of “Conspiracy Theories: Secrecy and Power In American Culture.”

After the assassination of President John F. Kennedy, Fenster pointed out, many civilians took to scouring photos of the Dallas motorcade route. Some unearthed significant clues, he said.

“There is a sense of powerlessness from events like this that I think invites us to try to make sense of who did it,” Fenster said.

For many, the appeal of sharing and scrutinizing images from before and ­after the tragedy, Fenster said, comes out of an honest desire to help.

Others may also be drawn to the idea of proving themselves smart and savvy, finding a nugget of information that no one else may have noticed and impressing others with their perceptiveness.

“It’s kind of like a fun video game,” Fenster said. “There’s a real excitement and energy involved in taking these photographs and connecting the dots. It’s like solving a puzzle.”

But some are concerned that the ­efforts will lead to unfounded accusations against innocent bystanders.

“So far, I’ve seen about 40 individuals labeled as ‘suspicious,’ ” wrote one commenter on Reddit. “By using a loaded term like that, people are already asso­ciating those individuals . . . with something nefarious. They’re not suspicious — they’re people watching the end of a marathon.”

The moderators of the Reddit comment thread tried to allay the concerns, by discouraging users from spreading the photos on Facebook and Twitter. Members, they said, simply wanted to help. Any clues identified, they said, would be forwarded to federal detectives. “We do not condone vigilante justice,” one user wrote. “Our aim is simply to provide tips for the FBI, not to take matters into our own hands.”

Fenster said that there can be real damage from premature speculations, as in the case of Richard Jewell, the ­police officer and Olympic security guard falsely accused of planting a bomb at the 1996 Summer Olympics in Atlanta.

Internet accusations can be fueled by prejudices based on race or religion.

But, Fenster said the phenomenon may, in some ways, be positive. With hundreds of photos of the crime scene before and after the blast, it is possible curious civilians may identify pieces of information that federal officials overlook, he said.

“To have all these eyeballs looking at these photographs is potentially enormously helpful,” Fenster said.

Still, others have taken to the Internet in an effort to prevent undue speculation. Hours after the bombing, word spread of a just-created website called BostonMarathonConspiracy.com.